Assassin's Creed: Freedom Cry

This trophy can be unlocked in any area in which there is a bell, such as plantations and de Fayet's compound (large red restricted area at Port-au-Prince. Bells are located on your in game HUD as a small white bell icon. In order for a guard to ring a bell you must first be spotted. A guard will then start running towards the bell to ring it. These individuals can be identified as the ones with a small red bell above their head. Let them get to the bell and ring it a few times before you kill them, as if you attack them to early it will not register and the trophy will not unlock.

Below is a video walkthrough by PowerPyx, that was recorded whilst playing Assassin's Creed IV: Black Flag: